A vehicle running on a curved path develops therein a centrifugal force, that is, a lateral acceleration corresponding to the running speed in the direction perpendicular to the direction of running.
In the case where the running speed is excessively high compared with the radius of curvature of the path, the wheels of the vehicle skid out of the path into the footway or the opposite lane, or in the worst case, the vehicle may even turn over. In order to prevent such a problem, it is common practice for the driver to decelerate the vehicle immediately before a curved path, followed by a gradual acceleration in what is called the "slow-in fast-out" driving technique. A curved path of which it is impossible to confirm the other end thereof, or what is called a "blind curve" or the like, however, often has a radius steadily decreasing. In such a situation, a very high driving technique is required.
On the other hand, certain types of vehicle tend to exhibit the trend of "understeering" in which the driving traces are increased in radius for the same steering angle if the vehicle is accelerated from a state of turning along a normal circle. In such a vehicle, it is necessary to increase the steering angle steadily with lateral acceleration. When the lateral acceleration exceeds a predetermined value (critical value) specific to the vehicle, however, the maneuvering is known to become difficult or impossible. A typical vehicle of such type includes a front-engine front-drive vehicle or what is called the F.F vehicle having maneuvering wheels doubling as driving wheels. In recent years, the trend has been more and more toward the F.F system, especially for passenger cars in order to maintain an advantageous position in the spaciousness of the cabin (underfoot space) or the like.
If the lateral acceleration is to be prevented from exceeding a critical value, it is essential that the driver know the radius of curvature of a curved path and adjust the driving force appropriately by accelerator pedal. For unskilled drivers, however, it is very difficult to finely control the amount of force on the accelerator pedal in face of a blind curve or the like.
In view of this situation, various types of driving force control systems have so far been suggested for reducing the driving force of a vehicle automatically before it becomes difficult or impossible to turn. Many of these systems are so constructed that the engine output or unit power is reduced in accordance with the magnitude of rolling of the vehicle body or the like without being interlocked with the amount of force on the accelerator pedal. Specifically, in view of the fact that a vehicle while turning always develops a rolling due to the lateral acceleration which increases with the running speed, the amount of rolling is detected by height sensors or the like mounted on the right and left sides of the vehicle body to reduce the engine output. In another system, the engine output is reduced by detecting the amount of irregular body swing, that is, yawing.
In the above-described driving force control system, after the rolling or the like phenomenon occurs, a TCL (Traction Calculate Unit) computes an optimum drive torque on the basis of the amount of rolling and an ECU (Electronic Control Unit) controls the output of the engine accordingly.
This control system, however, has the following disadvantage: In a situation where the rolling sharply increases, for example, the output control may be delayed, or what is called "hunting motion" is liable to occur in which the cancellation of control after a rolling may cause another rolling, thus requiring another output control.
Under these circumstances, a control system has been closely watched in which the driving force is controlled in accordance with a stability factor (a specific value determined from suspension and tire rigidity) as well as the running speed and steering angle. This driving force control system, in which a data associated with the moment of the driver operating the steering wheel is applied to the ECU, is capable of controlling engine output (in what is called an advance control) before occurrence of an excessive rolling or the like phenomenon. The steering angle indispensable for this control operation is usually based on the neutral position of the front wheels, that is, the steering shaft stored in a RAM (Random Access Memory). The displacement from this neutral position is detected by a steering angle sensor using a slit plate and a phototransistor mounted on the steering shaft, and is applied to the ECU.
The amount of steering for bringing the steering wheel to the position of full steering, that is, the number of revolutions required for lock-to-lock operation is several (generally, 2.5 to 3 revolutions). Even in the case where a slit or the like is formed at the neutral point on the slit plate of the steering angle sensor, therefore, a stationary steering operation with the battery or wiring removed (such as at the time of maintenance) often causes the neutral position to be displaced by one revolution from the normal state. Also, the neutral positions of the steering shaft and the front wheels naturally undergo a change when the gears in the steering unit are worn out or at the time of toe-in adjustment while the vehicle is under repair. As a result, the apparent angle of turning the steering wheel may sometimes differ from the actual steering angle in a minor order.
If the vehicle is driven under this condition, the output control fails to be effected as necessary, with the result that the vehicle falls into a dangerous situation or a small turn of the steering wheel may reduce the output, thus making it impossible for the driver to maneuver the vehicle by his or her own will.